14-Feb-2008

Introduction

The Interactive queue is a higher priority queue with the purpose of serving debugging sessions and interactive jobs.  The maximum number of CPUs that can be accessed through this queue is 16 with a maximum job walltime of 30 minutes.  This queue is available 24 hours per day. This queue will provide a mechanism for debugging your code, in that you will be given an interactive command line prompt on a compute node where you can run your job interactively. There is usually little or no wait time to get access to this queue. This queue is not intended for the purpose of running short compute jobs. It is for debugging sessions and for jobs that require interactive execution. The remainder of this document will describe how to use this queue.

Submitting an Interactive Job

The usual way to submit jobs to a queue is with a PBS batch script. However, in the case of the Interactive queue, you must use the qsub command line (rather than a PBS batch script) in combination with the -I argument to access this queue as illustrated in the following example:

The qsub arguments in this example represent the following options which you will recognize from your PBS batch scripts:

You can specify any PBS batch script argument on the qsub command line. Once this interactive job is submitted and begins to execute, you will receive a command line prompt on a compute node where you can manually run your code for debugging purposes as follows:

Note: In the example above, note that the hostname changed from a login node designation to a compute node designation. At this point you have a command prompt on an actual compute node.

Note: Do not use a PBS batch script to access the interactive queue. Use the qsub command line as illustrated above along with any PBS arguments you would normally use in a batch script. If you try to use a PBS batch script, the job submission will not work.

Submitting an MPI Interactive Job

If you need to submit a multiprocessor or multinode job to the Interactive queue, you will do so the same way as described above except that you will request a larger number of nodes and processors (ppn) as in the following example:

Running this command will give you two nodes, and two processors per node, in the interactive queue. You will be given an interactive prompt on one of those nodes where you can use mpiexec to launch a multiprocessor job for debugging. Your MPI code will have access to all of the nodes allocated to your job even though you have a command line prompt on only a single node. PBS and mpiexec will communicate with each other so that you have access to all of the nodes allocated to your job. See our FAQ on running MPI jobs with mpiexec.